doc/style.css

fncache: avoid loading the filename cache when not actually modifying it

With time, fncache can become very large. The mozilla-central repo for example,
has a 31M and growing fncache file. Loading this file takes time (280ms for the
mozilla-central repository).

In many scenarios, we don't need to load fncache at all. For example, when
committing changes to existing files, or pushing such commits to another clone.
This patch detects when a name is added via store.vfs(), and only loads the
cache if a) the data metadata file doesn't already exist, or b) when opening
for appending, the data or metadata file exists but has size  (a transaction
rollback leaves behind such files).

Benchmarks (run on Macos 10.13 on a 2017-model Macbook Pro with Core i7 2.9GHz
and flash drive), each test without and with patch run 5 times:

* committing to an existing file, against the mozilla-central repository.
  Baseline real time average 2.3736, with patch 1.9884.

* unbundling a large changeset consisting *only* of existing-file modifications
  (159 revisions, 1050 modifications, mozilla-central
  4a250a0e4f29:beea9ac7d823), into a clone limited to the ancestor revision of
  that revset). Baseline real time average 1.5048, with patch 1.3108.

/*
 * Styles for man pages, which match with https://mercurial-scm.org/
 *
 * Color scheme & layout are borrowed from
 * https://mercurial-scm.org/css/styles.css
 *
 * Some styles are from html4css1.css from Docutils, which is in the
 * public domain.
 */

body {
    margin: 0;
    padding: 0;
    font-family: sans-serif;
}

.document {
    position: relative;  /* be a top of absolute positioning */
    margin: 1.5em 1.8em;
    padding: 0;
    line-height: 1.3;
}

/* layout: toc to right */
#contents {
    position: absolute;
    right: 0;
    top: 0;
    width: 26%;
}

/* layout: others to left */
h1.title, h2.subtitle, .section { width: 72%; }
.section .section { width: auto; }
table.docinfo { max-width: 72%; }

/* headings */
h1, h2, .topic-title, .admonition-title {
    font-family: "MgOpen Cosmetica", "Lucida Sans Unicode", sans-serif;
    font-weight: normal;
}
h1, h2, .topic-title, .admonition-title {
    margin: 1em 0 0.5em;
}
h1.title { font-size: 300%; }
h2.subtitle, h1 { font-size: 200%; }
h2, .topic-title, .admonition-title { font-size: 140%; }

/* subtitle starts with lowercase in man pages, but not in HTML */
h2.subtitle:first-letter { text-transform: uppercase; }

/* override first/last margin */
.first, h1.title, h2.subtitle { margin-top: 0 !important; }
.last, .with-subtitle { margin-bottom: 0 !important; }

blockquote, pre, dd .option-list, .field-list {
    margin: 0.2em 0 1em 2em;
}

kbd, tt, pre { font-family: monospace; }

dt { font-weight: bold; }
dd { margin-bottom: 0.5em; }

th, td { padding: 0.1em 0.2em; border: 0 none; }
th { font-weight: bold; text-align: left; }

a:link, a:visited { text-decoration: underline; }
a:hover, a:focus { text-decoration: none; }
a:link { color: #00b5f1; }
a:visited { color: #5c9caf; }
a:link.toc-backref, a:visited.toc-backref {
    text-decoration: none;
    color: inherit;  /* NOTE: `inherit' is not supported by IE6 */
}

div.admonition, div.attention, div.caution,
div.danger, div.error, div.hint, div.important,
div.note, div.tip, div.warning {
    border-top: 1px #ccc solid;
    border-bottom: 1px #ccc solid;
    padding: 0.3em 1em;
    margin: 1em;
}

div.note {
    border-color: #fcc200;
}


/*
 * The following styles are from Docutils.
 * Please refine if necessary.
 */

table.borderless td, table.borderless th {
    /* Override padding for "table.docutils td" with "! important".
       The right padding separates the table cells. */
    padding: 0 0.5em 0 0 ! important;
}

.hidden {
    display: none;
}

blockquote.epigraph {
    margin: 2em 5em;
}

div.abstract {
    margin: 2em 5em;
}

div.dedication {
    margin: 2em 5em;
    text-align: center;
    font-style: italic;
}

div.figure {
    margin-left: 2em;
    margin-right: 2em;
}

div.footer, div.header {
    clear: both;
    font-size: smaller;
}

div.line-block {
    display: block;
    margin-top: 1em;
    margin-bottom: 1em;
}

div.line-block div.line-block {
    margin-top: 0;
    margin-bottom: 0;
    margin-left: 1.5em;
}

div.sidebar {
    margin: 0 0 0.5em 1em;
    border: medium outset;
    padding: 1em;
    background-color: #ffffee;
    width: 40%;
    float: right;
    clear: right;
}

div.sidebar p.rubric {
    font-family: sans-serif;
    font-size: medium;
}

div.system-messages {
    margin: 5em;
}

div.system-messages h1 {
    color: red;
}

div.system-message {
    border: medium outset;
    padding: 1em;
}

div.system-message p.system-message-title {
    color: red;
    font-weight: bold;
}

h1.section-subtitle, h2.section-subtitle, h3.section-subtitle,
h4.section-subtitle, h5.section-subtitle, h6.section-subtitle {
    margin-top: 0.4em;
}

hr.docutils {
    width: 75%;
}

img.align-left {
    clear: left;
}

img.align-right {
    clear: right;
}

ol.simple, ul.simple {
    margin-bottom: 1em;
}

ol.arabic {
    list-style: decimal;
}

ol.loweralpha {
    list-style: lower-alpha;
}

ol.upperalpha {
    list-style: upper-alpha;
}

ol.lowerroman {
    list-style: lower-roman;
}

ol.upperroman {
    list-style: upper-roman;
}

p.attribution {
    text-align: right;
    margin-left: 50%;
}

p.caption {
    font-style: italic;
}

p.credits {
    font-style: italic;
    font-size: smaller;
}

p.label {
    white-space: nowrap;
}

p.rubric {
    font-weight: bold;
    font-size: larger;
    color: maroon;
    text-align: center;
}

pre.address {
    margin-bottom: 0;
    margin-top: 0;
    font-family: serif;
    font-size: 100%;
}

pre.literal-block, pre.doctest-block {
    margin-left: 2em;
    margin-right: 2em;
}

span.classifier {
    font-family: sans-serif;
    font-style: oblique;
}

span.classifier-delimiter {
    font-family: sans-serif;
    font-weight: bold;
}

span.interpreted {
    font-family: sans-serif;
}

span.option {
    white-space: nowrap;
}

span.pre {
    white-space: pre;
}

span.problematic {
    color: red;
}

span.section-subtitle {
    /* font-size relative to parent (h1..h6 element) */
    font-size: 80%;
}

table.citation {
    border-left: solid 1px gray;
    margin-left: 1px;
}

table.footnote {
    border-left: solid 1px black;
    margin-left: 1px;
}

h1 tt.docutils, h2 tt.docutils, h3 tt.docutils,
h4 tt.docutils, h5 tt.docutils, h6 tt.docutils {
    font-size: 100%;
}

ul.auto-toc {
    list-style-type: none;
}

div.contents.local {
    -moz-column-width: 10em;
    -moz-column-gap: 1em;

    -webkit-column-width: 10em;
    -webkit-column-gap: 1em;
}